EXPERIMENTAL STUDY OF HEREDITY 63 



anything else, made them superior to all others that had 

 preceded. He carefully counted the number of plants 

 bearing each kind of seed, and found that the number 

 of smooth-seeded plants was to those with wrinkled 

 seeds as 3 : i. 



51. Theory of Purity of Gametes. When the wrinkled 

 seeds (one-fourth of the total crop) were sown they all 

 bred true to wrinkledness their descendants of the F 3 

 generation bearing only wrinkled seeds. The expression 

 was alike in every case. The gametes that united to 

 produce these plants were therefore considered pure for 

 "wrinkledness;" that is, it was inferred that they did not 

 carry any inheritance tending to produce smoothness of 

 seed. 



52. Not All Dominants Alike. But when the seeds of 

 the F 2 plants, having only smooth seeds, were sown it 

 was found that the dominants were not alike, except in 

 external appearance. The seeds, though all appeared 

 smooth, carried different inheritances. One-third of 

 them (i.e., one-fourth of all the seed produced by the F 2 

 generation) bred true to smoothness, being therefore pure, 

 or homozygous for smoothness; the other two-thirds of 

 the dominants (i.e., one-half of all the seed produced) 

 again segregated in the ratio of 3:1 one-fourth wrinkled 

 and three-fourths smooth, showing that they were hetero- 

 zygous; that is, that they still carried inheritance from 

 both the wrinkled and smooth-seeded grandparents. 



If we designate the first parental generation by P, the 

 dominant character (whatever it may be) by D, and the 

 recessive character by R, then the facts above described 

 may be diagrammed as follows : 



